14 research outputs found
Quasiperiodic oscillations in a strong gravitational field around neutron stars testing braneworld models
The strong gravitational field of neutron stars in the brany universe could
be described by spherically symmetric solutions with a metric in the exterior
to the brany stars being of the Reissner-Nordstrom type containing a brany
tidal charge representing the tidal effect of the bulk spacetime onto the star
structure. We investigate the role of the tidal charge in orbital models of
high-frequency quasiperiodic oscillations (QPOs) observed in neutron star
binary systems. We focus on the relativistic precession model. We give the
radial profiles of frequencies of the Keplerian (vertical) and radial epicyclic
oscillations. We show how the standard relativistic precession model modified
by the tidal charge fits the observational data, giving estimates of the
allowed values of the tidal charge and the brane tension based on the processes
going in the vicinity of neutron stars. We compare the strong field regime
restrictions with those given in the weak-field limit of solar system
experiments.Comment: 26 pages, 6 figure
Photon capture cones and embedding diagrams of the Ernst spacetime
The differences between the character of the Schwarzschild and Ernst
spacetimes are illustrated by comparing the photon capture cones, and the
embedding diagrams of the sections of the equatorial planes
of both the ordinary and optical reference geometry of these spacetimes. The
non-flat asymptotic character of the Ernst spacetime reflects itself in two
manifest facts: the escape photon cones correspond to purely outward radial
direction, and the embedding diagrams of both the ordinary and optical geometry
shrink to zero radius asymptotically. Using the properties of the embedding
diagrams, regions of these spacetimes which could have similar character are
estimated, and it is argued that they can exist for the Ernst spacetimes with a
sufficiently low strength of the magnetic field.Comment: 12 pages, 7 figure
Optical reference geometry of the Kerr-Newman spacetimes
Properties of the optical reference geometry related to Kerr-Newman
black-hole and naked-singularity spacetimes are illustrated using embedding
diagrams of their equatorial plane. Among all inertial forces defined in the
framework of the optical geometry, just the centrifugal force plays a
fundamental role in connection to the embedding diagrams because it changes
sign at the turning points of the diagrams. The limits of embeddability are
given, and it is established which of the photon circular orbits hosted the by
Kerr-Newman spacetimes appear in the embeddable regions. Some typical embedding
diagrams are constructed, and the Kerr-Newman backgrounds are classified
according to the number of embeddable regions of the optical geometry as well
as the number of their turning points. Embedding diagrams are closely related
to the notion of the radius of gyration which is useful for analyzing fluid
rotating in strong gravitational fields.Comment: 28 pages, 17 figure
Summary of session A4 at the GRG18 conference: Alternative Theories of Gravity
More than 50 abstracts were submitted to the A4 session on "Alternatives
Theories of Gravity" at the GRG18 conference. About 30 of them were scheduled
as oral presentations, that we summarize below. We do not intend to give a
critical review, but rather pointers to the corresponding papers. The main
topics were (i) brane models both from the mathematical and the
phenomenological viewpoints; (ii) Einstein-Gauss-Bonnet gravity in higher
dimensions or coupled to a scalar field; (iii) modified Newtonian dynamics
(MOND); (iv) scalar-tensor and f(R) theories; (v) alternative models involving
Lorentz violations, noncommutative spacetimes or Chern-Simons corrections.Comment: 9 pages, no figure; the GRG18 conference was held in Sydney,
Australia, 8-13 July 200
Appearance of Keplerian discs orbiting Kerr superspinars
We study optical phenomena related to appearance of Keplerian accretion discs
orbiting Kerr superspinars predicted by the string theory. The superspinar
exterior is described by the standard Kerr naked singularity geometry breaking
the black hole limit on the internal angular momentum (spin). We construct
local photon escape cones for a variety of orbiting sources that enable to
determine the superspinars silhouette in the case of distant observers. We show
that the superspinar silhouette depends strongly on the assumed edge where the
external Kerr spacetime is joined to the internal spacetime governed by the
string theory and significantly differs from the black hole silhouette. The
appearance of the accretion disc is strongly dependent on the value of the
superspinar spin in both their shape and frequency shift profile. Apparent
extension of the disc grows significantly with growing spin, while the
frequency shift grows with descending spin. This behavior differs substantially
from appearance of discs orbiting black holes enabling thus, at least in
principle, to distinguish clearly the Kerr superspinars and black holes. In
vicinity of a Kerr superspinar the non-escaped photons have to be separated to
those captured by the superspinar and those being trapped in its strong
gravitational field leading to self-illumination of the disc that could even
influence its structure and causes self-reflection effect of radiation of the
disc. The amount of trapped photons grows with descending of the superspinar
spin. We thus can expect significant self-illumination effects in the field of
Kerr superspinars with near-extreme spin .Comment: accepted in Classical and Quantum Gravity (September 1st, 2010